Various vehicle control systems for controlling the speed of a vehicle are proposed (for example, U.S. Pat. No. 5,928,299 which corresponds to JP 3432881). A vehicle control system disclosed in U.S. Pat. No. 5,928,299 identifies the distance (estimated distance) required for a controlled vehicle to stop within a predetermined time when it is decelerated from a vehicle speed at predetermined deceleration rate. Then, the control system sets a reference node in a preliminary controlled vehicle position located ahead the position of the controlled vehicle by the estimated distance and also sets another reference node in a position located ahead the reference node by a distance (vehicle speed×predetermined time). The control system also obtains a passable vehicle speed at which the vehicle can travel the distance between the two reference nodes at acceleration rate equal or lower than reference lateral acceleration rate. When it is determined that there is a curve on the road ahead the vehicle and the vehicle speed is in the excess of the passable vehicle speed, automatic deceleration is executed by a vehicle speed control unit to ensure that the vehicle can pass through the curve safely.
This vehicle control system thus identifies a passable vehicle speed at which a vehicle can travel at acceleration rate equal to or lower than reference lateral acceleration rate. The passable vehicle speed can become higher or lower than a vehicle speed at which the vehicle can actually travel depending on the setting of the reference lateral acceleration rate. Therefore, the reference lateral acceleration rate must be set adequately, and vehicle speed control cannot be executed in accordance with actual road conditions and the state of the vehicle unless the reference lateral acceleration rate is adequately set.
The above vehicle control system however only determines a passable vehicle speed at which the vehicle can pass the curve at acceleration rate equal to or lower than the reference lateral acceleration rate. It cannot execute vehicle speed control taking the feel of danger that drivers will have on a curving road into account.